The invention is in the field of packaging containers, and methods and apparatus for fabricating same.
A variety of packaging containers constituted by bottom sections and top sections made of thermo-plastic resin sheet are extensively employed for packaging fruits, foodstuffs such as eggs, and other articles. These packaging containers are obtained by sealing the bottom sections and the top sections, respectively, in accordance with various methods.
Typically the top section is provided with an undercut portion that is employed for fitting the top section to the bottom section. However, since the maximum amount of undercut allowable is of the order of 0.8-1.0 mm, the fastening strength is relatively small, and therefore the top section is liable to be removed from the bottom section during handling or transportation.
Furthermore, a method has been proposed recently for manufacturing packaging containers, having a top section, made of a heat shrinkable sheet, fitted to a bottom section by the utilization of its heat shrinking property. However, the latter method has certain disadvantages. The material of the top section is limited to resin sheets having a particular heat shrinking property and when the top section is fitted to the bottom section, sometimes the heat shrinking is not sufficiently and uniformly effected, resulting in insufficient fastening of the top and bottom section, and the possibility that the top section will come off from the bottom section.
In order to overcome the latter disadvantage a packaging container has been developed having the top section and the bottom section made of thermoplastic sheet with flange sections at the respective peripheries. The flanges are strongly joined together by heat-sealing. As a result, the top section will not come off the bottom section during handling or transportation, and such packaging containers are popularly employed as one-way-use packaging containers.
The latter packaging containers still suffer from certain drawbacks. It is difficult to remove the top section from the bottom section, and therefore a knife must be used or an opening means must be provided along with each of the packaging containers, as in the case of U.S. Pat. No. 3,352,479. If a knife is used to cut the packaging container, the contents may be damaged. If an opening means is provided with the packaging container, the appearance thereof will become worse, and the total cost of the packaging container will be increased.
U.S. Pat. No. 3,443,741 discloses a packaging container in which a lid member is secured on a bottom section having a radially outwardly directed flange adjacent its open upper end.
The flange and the lid member form polygonal exterior margins which are related to each other in such a manner as to provide a plurality of tear tabs from the lid member, thereby facilitating the removal of the lid member from the bottom sections. This prior art is also disadvantageous in that, when the lid member is placed on the bottom section, the lid member slips out of place and if the tear tab formed by the lid member is kept in a horizontally extended state, the packaging containers will take up an excessive amount of horizontal space. Accordingly this prior art is not suitable for packing and displaying a number of packaging containers.
U.S. Pat. No. 3,491,935 discloses a method in which peelable sealing is applied to a packaging container so that the top section can be readily peeled off the bottom section. In this method, the top section and the bottom section are made of particular thermo-plastic resin sheets, and the softening point of the resin forming the top section is lower than that of the resin forming the bottom section. Furthermore a peelable seal is provided without melting the bottom section when the top section and the bottom section are heated and pressed together. Therefore, this method has the advantage that the top section can be readily peeled off the bottom section. However, this method also involves difficulties. Since the method is applicable to particular resin sheets, the range of application of the method is limited. In addition, the flange section of the bottom section is extended horizontally, the margin of the top section is flat, and the heat-sealed portions are horizontally extended. Accordingly, if the carton is filled with heavy contents, it is liable to be folded. That is, the strength of the carton according to the latter method is insufficient for practical use.
A packaging container in which the bottom section is made of foamed plastic material, while the top section is made of non-foamed plastic material, is disclosed by U.S. Pat. No. 3,424,363. The bottom section and the top section are hingedly connected together by heat-sealing them. The packaging container is opened by turning the top section around the heat-sealed portion, and a locking means is associated with the bottom and top sections along the side of the container. A disadvantage is that the temporary locking means is unlocked during handling. Moreover, if the weight of the contents is of the order of 500-1000 g, as in the case of a packaging container including 10-12 eggs, the packaging container is sometimes folded or bent approximately perpendicularly to its londitudinal direction. Thus, the strength of this packaging container is also inadequate.